In air processing devices known from the prior art air dryer cartridges are used for extracting humidity contained in the pressurized air supplied by the compressor. Due to the fact that the desiccant used in the air dryer cartridges only has a limited capacity for the humidity (and other particles, oil droplets or contaminations), after a specific time of use of an air dryer cartridge in the load phase a regeneration of the air dryer cartridge is required. During the regeneration air previously dried streams in reverse direction through the air dryer cartridge for extracting humidity or other contaminations from the desiccant of the air dryer cartridge. Accordingly, the input port of the air dryer cartridge during the load phase is connected with the compressor, whereas the same port during the regeneration is connected with a de-aerating port. However, the intermittent load phases and regeneration phases of an air dryer cartridge have the consequence that a permanent supply of dried pressurized air is not possible. Instead, the supply of dried pressurized air is interrupted for the time of the regeneration.
For avoiding these interruptions, air processing devices are known having two (or more) air dryer cartridges. For these air processing devices it is possible to regenerate one of the two air dryer cartridges while using the other of the air dryer cartridges for a simultaneous load phase. Accordingly, it is possible to permanently supply pressurized air from the compressor. For these air processing devices the stream of dried pressurized air output from the air dryer cartridge used in the load phase is divided in two partial air streams. The first partial air stream is used for supplying the pressurized air system or circuits. The other partial air stream is used for the regeneration of the other air dryer cartridge and streams in reverse direction through the other air dryer cartridge for a regeneration of the same. However, in consequence throughout the whole use of the compressor only a partial air stream of the pressurized air supplied by the compressor is in fact supplied to the consumer circuits. Air processing devices with two air dryer cartridges are also called “twin towers”. By use of twin towers, the need might be overcome to activate and deactivate the compressor—instead the compressor might be permanently driven which leads to a simplification of the control of the compressor and/or of an increase of the lifetime of the compressor.
U.S. Pat. No. 5,685,896 suggests using a twin tower with a valve unit comprising two valve groups. Each of these two valve groups is related with one of the air dryer cartridges. The two valve groups in an alternating fashion are controlled into a switching state “load” and a switching state “regeneration”. By means of a pneumatic control pressure after a predetermined time both valve groups are switched to the other operating state. The outputs of the air dryer cartridges are connected via a kind of switching valve with the pressurized air system or circuits, wherein the valve element of the switching valve comprises a throttle bore for redirecting a dried partial air stream to the air dryer cartridge used in the regeneration state.
EP 0 933 117 A1 also discloses the use of a switching valve for dividing an air stream supplied by a compressor into two partial air streams. Also here one partial air stream is used for supplying the pressurized air system or circuits with pressurized air whereas the other partial air stream is used for a regeneration of the other air dryer cartridge.
Other air processing devices using switching valves for treating two partial air streams are disclosed in documents U.S. Pat. No. 5,901,459, U.S. Pat. No. 5,901,464 and U.S. Pat. No. 5,983,516.
U.S. Pat. No. 5,961,698 related to the removal of segregated particles or fluid from a twin tower. During the regeneration of an air dryer cartridge a cyclone and additional filter elements are used. The alternating use of the two air dryer cartridges is controlled by a solenoid valve which is switched by a timer.
DE 35 25 083 A1 discloses the supply of a pressurized air system comprising a multi protection valve linked with a central conduit with an integrated check valve to a compressor via a pressure controller and a switching valve. The switching valve is connected with two paths that are connected with the central conduit upstream from the check valve. In both paths respective air dryer cartridges are used. A 4/2-way solenoid valve by alternating switching controls two operating states. In the two operating states in alternating fashion one path is used for the load phase, whereas the other path is used for a regeneration. Check valves opening in load or supply direction are located in the paths, wherein the check valves are located downstream from the air dryer cartridges. Respective bypass conduits are provided for bypassing the check valves. Throttles are integrated into the bypass conduits. De-aerating valves are located upstream from the air dryer cartridges, wherein the de-aerating valves in an alternating fashion are pilot-controlled by the 4/2-way solenoid valve. Due to the pneumatic pilot control by the electronically controlled 4/2-way solenoid valve, the de-aerating valve in the path used for the load or supply phase is blocked so that pressurized air from the compressor is supplied via the pressure controller and the switching valve, the air dryer cartridge of this path and the check valve to the central conduit. Instead, at the same time the 4/2-way solenoid valve opens the de-aerating valve in the other path for a regeneration. A partial air stream of dried air from the path in the load phase streams via the bypass conduit and the throttle in reverse direction through the air dryer cartridge in the path being regenerated. With the dried air flowing in reverse direction humidity and contaminations from the air dryer cartridge to be regenerated are transferred via the de-aerating valve to the environment. An alternative embodiment disclosed in this document addresses the changing process for the use of one path for the load phase to the use of this path in the regeneration state. The document suggests using a two-pressure valve parallel to the switching valve, wherein an output of the two-pressure valve is connected with the 4/2-way solenoid valve. On the other hand, the document suggests to use two 3/2-way solenoid valves instead of the 4/2-way solenoid valve, wherein in this case the two 3/2-way solenoid valves are used for controlling the alternating load phases and regeneration phases in the two paths.
Also DE 32 44 414 A1 discloses the alternating use of two paths with associated air dryer cartridges for a load phase and a regeneration phase. At the input of the two paths a 4/2-way solenoid valve is located. In one switching state of the 4/2-way solenoid valve one path is connected with the compressor for the load phase, whereas the other path is connected with the de-aerating port. The partial air stream used for the regeneration is taken from the central conduit upstream from a check valve. This partial air stream after having passed a throttle valve flows through the air dryer cartridge in the path operated for regeneration streaming in reverse direction. For keeping the relation of the flow of the partial air stream in the path operated in the load state to the partial air stream in the path operated for regeneration independent on the delivery rate of the compressor the patent application suggests using a throttle valve with a continuously variable throttle characteristic. A change of the throttle characteristic is pneumatically controlled. As the control pressure for controlling the throttle characteristic the pressure at the output of the compressor is used.